1. FIELD OF THE INVENTION
The present invention relates to an apparatus and a method for separating a liquid mixture by pervaporation, and a vapor drying apparatus employing such a separating apparatus.
2. DISCUSSION OF BACKGROUND
As one of methods for separating liquid mixtures, so-called pervaporation is known wherein a certain specific component in a liquid mixture is separated by means of a membrane having a specific affinity to the component by placing the liquid mixture on one side (the primary side) of the membrane and vacuuming the other side (secondary side) by a vacuum pump or purging the secondary side with an inert gas so that the partial vapor pressure of the component passing through the secondary side is maintained to be lower than the equilibrium vapor pressure at the primary side.
This separation method is useful for separating a liquid mixture which can not be separated by a usual distillation method, such as an azeotropic mixture or a mixture of liquids having boiling points close to one another.
In the pervaporation method, the difference between the equilibrium vapor pressure at the primary side and the partial vapor pressure at the secondary side is desired to be as large as possible. For this purpose, it is advisable to maintain the liquid temperature on the primary side at a high level and to maintain the vacuuming degree on the secondary side at a high level. Various considerations are required in this connection.
A vapor drying apparatus may be mentioned as one of applications of such a liquid separating apparatus by pervaporation. The vapor drying apparatus is an apparatus for drying an article to be dried such as a semiconductor wafer after washing it with water. As described, for example, in Japanese Unexamined Patent Publication No. 106630/1987, the vapor drying apparatus comprises a treating tank designed to carry out drying by condensing vapor of a drying liquid comprising a water-soluble organic solvent as the main component, on the surface of the article to be dried after washing with water, and a means for purifying a condensed liquid discharged from the treating tank. In the treating tank, a section for generating the vapor of the drying liquid, a dry space section for placing the article to be dried, and a receiving section for collecting a condensed liquid below the dry space section, are provided, and the collected condensed liquid will have water removed by a purifying means and then will be returned to the treating tank.
The liquid separating apparatus by pervaporation is used as this purifying means.
In the vapor drying of e.g. a semiconductor wafer, it is necessary to maintain the solvent concentration in the drying liquid at a high level of e.g. at least 93% to prevent a defect such as a water mark. To purify the above-mentioned condensed liquid or the drying liquid having water included to such a purity level, it is preferred to provide a plurality of membrane module units and to sequentially conduct the purification depending upon the purity of the liquid to be regenerated. However, when purification is conducted by using a plurality of membrane module units, there is a problem such that depending upon the manner of supplying the liquid to be treated to each membrane module unit, the load for treatment to the membrane module unit tends to be large, and the operation efficiency tends to be low.
Further, the vapor drying apparatus is a batch system, whereby supply of the liquid to the liquid separating apparatus varies substantially, and various considerations are required to maintain the separation performance to be constant.